An FM-CW radar using a millimeter wave, which is one of electric waves, for use as a radar mounted on a vehicle has had a problem of an interference with a radar mounted on an object vehicle. That is, as shown in part (A) of FIG. 4, when a subject vehicle MM on which the type of radar to scan a beam in the azimuthal direction is mounted and an object vehicle OM1 face each other, spike noise is superimposed (mixed) on a beat signal at a timing when the beat is generated between a reception signal directly received from the object vehicle OM1 and a transmission signal of the subject vehicle MM. As shown in part (B) of FIG. 4, when the subject vehicle MM on which the type of radar to perform beam scanning is mounted and an object vehicle OM2 on which a monopulse radar is mounted face each other, spike noise is also superimposed on a beat signal obtained from a transmission signal and a reception signal. Furthermore, as shown in part (C) of FIG. 4, when an object vehicle OM4 traveling in front of the subject vehicle MM and an object vehicle OM3 that transmits an electric wave to the object vehicle OM4 exist, a signal that is transmitted from a radar mounted on the object vehicle OM3 and then reflected on the object vehicle OM4 is superimposed on a reception signal received by a radar of the subject vehicle MM. Accordingly, spike noise is superimposed on a beat signal.
A method for detecting such spike noise is disclosed in
Patent Document 1.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 6-160512
In the method disclosed in Patent Document 1, after a frequency spectrum of a beat signal is determined using FFT, the presence or absence of an interference is detected by determining whether a noise floor level of the frequency spectrum is increased or not. When it is determined that an interference exists, a transmission frequency is changed. The change of the transmission frequency is repeated until no interference is detected (the influence of the interference becomes negligible).
However, because an FM-CW radar requires a wide frequency band for frequency modulation, the frequency change is substantially difficult in terms of oscillator characteristics or legislation. Additionally, a target cannot be detected while the frequency change is performed. Furthermore, for example, because repetitive periods of scanning a beam in the azimuthal direction becomes unequal due to the performing of the frequency change, a function of examining the correlation between frequency spectrums of beat signals for beams in the same azimuth and tracing a target with recognition of targets having a strong correlation as a single target is also impeded. That is, the target trace is interrupted every time the frequency change is performed.